Equipped libdax_msgs with reference counter. Completed its mutex protection.

libburn/trunk/libburn/libburn.h

@@ -1877,9 +1877,8 @@ int burn_msgs_obtain(char *minimum_severity,
 
 /* ts A70915 */
 /** Replace the messenger object handle of libburn by a compatible handle
-    obtained from a related library. The own message object gets destroyed,
-    so it is IMPORTANT to shut down libburn BEFORE the library which provides
-    the messenger object. See also: libisofs, API function iso_get_messenger().
+    obtained from a related library. 
+    See also: libisofs, API function iso_get_messenger().
     @param messenger The foreign but compatible message handle.
     @return 1 : success, <=0 : failure
 */